1. Field of the Invention
The present invention relates to a process for producing polycyanoaryl ether powder. More particularly, it is concerned with a process for the efficient production of a polycyanoaryl ether powder which is useful as a material for electronic appliances, electric appliances, and parts of various machines.
2. Description of Related Arts
Heretofore, polycyanoaryl ether produced from dichlorobenzonitrile and dihydric phenol has been known as high performance engineering plastics (see Japanese Patent Application Laid-Open No. 223226/1987). In order to increase the molecular weight of said polycyanoaryl ether, a process for adding difluorobenzonitrile was proposed (Japanese Patent Application Laid-Open No. 189435/1988).
Polycyanoaryl ether produced from dihalogenobenzonitrile and dihydric phenol according to these processes is a crystalline polymer, so it is solidified by cooling after the completion of polymerization. In order to put said polymer into practical use, however, it is required to remove the solvents, inorganic salts, oligomer and the like. Their removal requires washing with solvents having low boiling points or water. For example, Japanese patent Application Laid-Open No. 212430/1987 discloses a method of purifying the above-mentioned polymer by treating with a specified solvent. For washing with solvent, however, a grinding step to mechanically grind the solidified polymer to fine powder is required, and said grinding step has defects that a large amount of energy is required and that it is difficult to obtain a sufficient purification-effect owing to the relatively large particle size of the polymer.
Under such circumstances, the present inventor's group developed a process wherein solvents such as methanol, ethanol, methyl ethyl ketone, acetone, or water are used as so-called pulverization solvents after dihalogenobenzonitrile and dihydric phenol are polycondensed (Japanese Patent Application Laid-Open No. 135833/1989). According to said process, fine particles can be obtained easily without a step to mechanically grind the resultant polymer, but in some cases, filtration ability after the pulverization is poor. And in other cases, solidifying phenomena occurs at the pulverization, which makes the polymer treatment inconvenient, and makes the filtration and washing as the subsequent steps difficult.
Further, if water is used solely as a pulverization solvent, solidification occurs locally which makes filtration and washing as the subsequent steps very difficult.
Polymers particulated by adding a pulverization solvent and ground by cooling to setting have a small bulk density and have a large volume of cake in washing, so the operation ability in the washing step and the granulating step is problematic. In addition, the liquid content in washing is so high that it takes a long time to remove the impurities and purify the product.
Further, methods of using N-methylpyrrolidone as a pulverization solvent has been studied, but, the powder obtained by pulverization step with a mixed solvent of N-methylpyrrolidone and water have difficulties in handling because of a large volume of cake in the washing step, and is low in bulk density. In the pulverization step with the use of the above-mentioned pulverization solvent, inconveniently a large amount of solvent must be used so as to prevent caking or adhering. Because of a small bulk density, flow rate becomes unstable when molding.
On the other hand, if solvent is distilled away without depositing the polymer, during or after the polymerization, flooding will occur, and caking and adhering occur. In this case, there are defects that the handling and washability become worse, and that bulk density is low.
In this situation, the present inventors repeated intensive studies to develop a process to dissolve the above-mentioned technical defects, and to obtain a polymer having a small liquid content and a good washability, with a small volume of cake in the washing step of the polymer obtained after the polycondensation of dihalogenobenzonitrile and dihydric phenol, and to produce stably and easily a polycyanoaryl ether powder which is high bulk density and is excellent in the moldability.
As the result, it was found that the above object can be attained by distilling away the solvent with a particular step after the polycondensation reaction. The present invention was accomplished on the basis of such knowledge.